222 CLINICAL APPLIED ANATOMY. 



When atrophy involves the extensor muscles at the back of 

 the neck the head falls forwards until the chin rests on the 

 chest. 



TABES DORSALIS. 



The afferent or sensory system of the spinal cord and those 

 portions of the posterior roots which intervene between the 

 posterior root ganglia and the point of entry of the roots into 

 the cord, are the parts which are principally involved in tabes 

 dorsalis. The lumbar region and the cauda equina are usually 

 the sites of the most advanced disease. The peripheral nerves 

 of the body may also suffer in varying degrees, and certain of 

 the cranial nerves, both motor and sensory, may also be invaded. 

 Consideration of the course taken by the fibres of a posterior 

 spinal root on entering the cord shows that the tabetic lesions 

 in the posterior columns correspond to the distribution of the 

 posterior root fibres. (Fig. 19.) 



From a posterior root three sets of intraspinal fibres may be 

 derived. The first set consists of fine fibres, which pass directly 

 to form Lissauer's tract at the tip of the posterior cornu of the 

 grey matter, and end in the substantia gelatinosa of that cornu. 

 The second set of fibres form the cornu-radicular zone. Some of 

 these pass directly forwards to become connected with the motor 

 cells of the anterior cornu, the rest do not terminate in the seg- 

 ment at which they entered the cord, but pass upwards for a 

 short distance, being gradually displaced forwards by the corre- 

 sponding fibres of the roots which enter above them. They are 

 thus pushed into the anterior and middle parts of the postero- 

 external column, or column of Burdach, and ultimately come 

 into connexion with the cells of Clarke's column, which in turn 

 give origin to the fibres of the direct cerebellar tract. The third 

 set consists of long fibres, which soon pass from the postero- 

 external column into the postero-internal column of Goll, in 

 which they are continued up to the nucleus of the funiculus 

 gracilis in the medulla. The long fibres which ascend in Goll's 

 column are principally derived from the posterior roots of the 



